Your guide to buying
a range hood

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Range Hood Power

The next step in this range hood buying guide will be to pick the amount of power of your future range hood, which will narrow your choices down considerably. Why power? Because it's probably the single most important feature of a range hood. That's why this part will be the most extensive part of this guide. You can consider it as a mini range hood CFM guide.

Sadly, range hoods are generally made with a set amount of power, which means that you can't pick a certain model and consequently pick the amount of power for it. You will have to pick the amount of power you will need beforehand and pick a range hood model which fits that specific amount. Now, a lot of us are immediately tempted to order the most powerful one. How impressive would it be to have a fancy commercial grade exhaust vent above our stove, right? Not only is it that a really bad idea, it’s actually dangerous! (More on that later.) Let's find out how much power your future range hood will need.

The power of a range hood is measured by the speed it can suck in the air (also called airflow displacement). This speed is measured in something called cubic feet per minute, abbreviated as CFM. We know that sounds a bit abstract, so to visualize it: CFM is the amount of cubes of air (of a foot high, wide and long) the range hood could suck in per minute. The higher the CFM of a range hood, the more cubes of air it can suck in during one minute, and therefore the more powerful the range hood is. 

Remember this CFM abbreviation, because you will need it later on. Remember this CFM abbreviation, because you will need it later on.

Power arm

Step 3.1 How to choose the amount of power - Ductwork

If you’re going to pick a ductless range hood, it couldn’t be simpler: more power is better. If that’s the plan, you can skip the coming part and continue to the last page of the guide (click).

If you’re going to pick a ducted range hood, however, it will be a bit trickier. Why? Because a ducted range hood is dependent on other parts of your kitchen. That’s why we need to consider them when choosing the amount of range hood power. And by following this range hood buying guide, you will do exactly that. To give you an overview of what we're going to do:

  • Check your ductwork (Step 3.1)
  • Check your cooking stove (Step 3.2)
  • Check the size of your kitchen (Step 3.3)

Even the 'doing' part of this range hood buying guide might be a bit trickier than the rest, it's still really simple. So, don’t worry. We will guide you through every step of the way. By the end of this guide you will have a general idea of how much power your future range hood will need. Let's start with checking your ductwork.

Notice

The information below is general information. ALWAYS check the manufacturer's recommendations with regards to duct size, length and so forth, as it supersedes the information below. Going against the manufacturer's recommendation might not just result in malfunctioning equipment, you might also risk voiding their warranty.

What kind of ductwork does your house have?

Simply said, we are going to check the ductwork which is already present in your house, because the amount of power your future range hood needs will be largely dictated by the ductwork in your house. That means that we are going to answer the following 3 questions:

  • What is the diameter of your duct?
  • How long is your duct?
  • How many bends does your duct have?

Duct size

One of the most important things to know when you're looking for a new range hood is your duct size, a.k.a. the diameter of the duct. Why? It so happens to be that you can only move a certain amount of air through any given space. This is also true for the duct you’re going to connect your range hood to. If you're trying to push too much air through your duct, you will hit resistance.

You can compare it blowing air through a drinking straw. It's easier to blow air through a thick straw compared to a thin one. The thinness of the drinking straw (among other things) will determine how much air you can blow through the straw at the same time. The resistance you will feel while blowing the air through the straw is something called static pressure. So, to get the air from one side of the straw to the other quickly, you will have to overcome 

this static pressure by blowing harder. Blow too hard and your head will turn red, you'll become dizzy and get a headache.

blow straw

The same thing happens in your duct as well. The more resistance inside of your duct, the lower the airflow will be. This airflow restriction will instantly reduce the CFM (power) of your range hood. This means that even if you have really powerful range hood, when connected to a relatively small duct, its output will effectively be bottlenecked by the duct size. It might even damage your range hood by causing overheating or mechanical failure, because the resistance within your duct will make your range hood blow have to blow a lot harder than usual to push air through the duct. On the other hand, if your duct is too big relative to the amount of CFM your range hood has, the velocity of the airflow might be too low.

Now, you might think "that's nice, but how will I get to know my duct size?" No worries, we'll lay it out for you below. Before we do, please read the tip below.

Tip

The coming part will have you measure the diameter of your duct. Before you start with the actual measuring, be sure to read the part after it about the length of your duct and the amount of bends it may contain, as it will also require you to do some measuring. That way you can measure everything in one go!

How to find your duct size

The easiest way to find your duct diameter is to look it up. If you've bought your house, look it up in your house plan (blueprints). If you're renting, check with your landlord.

PDF

Question 4: If you already know your duct size, go straight to question 4.2 (skip question 4.1) on the PDF and write down your duct size in inches.

Another option is to measure it yourself, which is really easy to do. All you will need is a tape measure, a calculator, a pen and a piece of string. If you happen to have one of those flexible plastic tape measures, simply wrap it around your duct and measure its circumference (without needing a string), but since most of us will have one of those self-retracting-and-hurt-your-hand kind of tape measures, we will have to use the string as these aren't flexible enough to tightly wrap around a round surface, it's wiser to use a piece of string. Take the string and tightly wrap it around the duct once and mark the string with the pen at the spot where the end of the string meets the other end. Measure the string up to the marked part. This is the circumference of your duct. Write down this dumber in inches.

Duct measuring

Wrap the string around the circumference
of the duct and mark it with a pen.

PDF

Question 4.1: Write down your duct circumference in inches. Your duct size will automatically be calculated at question 4.2. If needed, round it up/down to the nearest round number.

Duct Diameter

The green arrow is the diameter.

To calculate the duct size, all we will have to do is to take your duct circumference which you've written down and divide it by 3.14, also known as π (pi). The answer you will get is the diameter of your duct. If you're using our PDF, after you've written down your duct circumference at question 4.1, the duct size will automatically be calculated for you. For example, let's say you've measured the circumference of your duct with a piece of string, which turned out to be 18.84 inches. When we 

divide it by 3.14, we will get a diameter of (18.84 / 3.14 =) 6 inches. Write it down. Who says you won't use math in real life? Be sure to not throw away your duct size answer, because later on (after finishing this guide), when you're looking for actual range hoods, you will need to know your duct size. Every range hood manufacturer will state the range hood's duct size in their product description.

Compare your duct size with the ones in the table below to get an indication of the amount of CFM you will need. (That said, always check the range hood manufacturer's recommendation with regards to the duct size.) If your duct size isn't a nice rounded number, just round it up/down to the nearest round number. E.g. 6.2 inch will be a 6 inch duct size.

Power (CFM)

Minimum
diameter duct (inches)

Recommended 
diameter duct (inches)

0-400

4

6

400-600

6

8

600-900

8

10

900-1200

10

12

1200+

12 *

12+ *

* Depends on the length of the duct, the amount of bends, the static pressure and the type of range hood motor (axial fan vs centrifugal fan).

PDF

Question 4.3: Write down the CFM ranges corresponding to your minimum diameter duct and your recommended diameter duct.

For example, let’s say your duct is 6 inches in diameter. When we look at the table above, we see that we could technically go for either a 0-400 CFM range hood (recommended diameter duct), or a 400-600 CFM range hood (minimum diameter duct). Write down both CFM ranges and read on!

How long is your duct?

We can be very clear about the length of your duct: the shorter the better. That's because the longer the duct, the more static pressure you will have. To come back to the straw example: it's much easier to blow air through a short straw compared to a long one. 

Generally, your ductwork shouldn’t exceed 50 linear feet (linear, as in, a straight line). However, the recommended maximum duct length can differs per range hood manufacturer. Some might have a lower or higher maximum linear duct length than others. What you're going to do after you finish reading this page of the guide, is to find out the total length of your duct. This means that you might have to look it up in your house plan (like with the duct size), ask your landlord or bust out your tape measure and measure it yourself. Please note that we're talking about a smooth ducts. We want to advice against any other ducts, as they will also cause additional resistance in your duct.

PDF

Question 5.1: Write down your total duct length.

How many bends does your duct have?

Straws

Which one do you think will be easier to blow air through?

However, the length is not the only important feature of the duct. Equally important are the amount of bends in your duct, also known as elbows. Every bend in your duct will raise the static pressure by adding little bit of resistance. And, as we’ve read before, static pressure is the resistance air encounters when its traveling through your duct. So, the more resistance you have in your duct, the lower the airflow will be.

Remember when you were a kid and you had one of those silly straws, which were bent in a all kinds of weird shapes? Drinking through such a straw took a lot more effort compared to drinking through a normal straight one. Well, the same happens when you blow air through a duct which has a lot of bends in it. It takes more power to push air through a duct with lots of bends in it compared to relatively straight duct. Therefore, the less amount of bends your duct contains, the better.



Not every house will be constructed in such a way that your duct runs from your kitchen to your roof/side wall in one straight line. That means your duct will most likely contain a certain amount of bends. Each 90 degree bend has so much impact on the 

45 degree bend

Duct elbow bend

90 degree bend

airflow, resistance-wise it would be as if you would add 10 linear feet of duct to the total duct size. Each 45 degree bend will be as if you would add 5 linear feet of duct to the total duct size. And that is exactly what we're going to do: count every bend in your duct. Add 10 linear feet per 90 degree bend to your total duct size, and add 5 linear feet per 45 degree bend to your total duct size.

PDF

Question 5.2: Count and write down the amount of 45 and 90 degree bends in your duct.

To sum up

  • Measure the diameter of the duct
  • Measure the length of the duct
  • Count the amount of bends in the duct. For every 45 degree bend, add 10 feet to the length of the duct you’ve measured at the previous point. For every 90 degree bend, add 10 feet of length.
  • Write down both the diameter and the length (including the length added by the bends).

Even though knowing the length of your duct won't directly give you a specific CFM number, it's still crucial to know. Similar to the duct size, you will have to compare this information with the recommended maximum duct length.

(This guide will be continued on the next page.)

Next page:
How much power does your stove have?

On the next page we will read about how much range hood power you will need based on your cooking stove, which is a very easy, but essential thing to do. Read on!

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